Digitally modified resistive output for a temperature sensor

ABSTRACT

A method for digitally controlling the resistive output of a temperature probe is disclosed. The system is comprised of a temperature sensor, a processor and a means under the control of the processor for modifying the resistive output such as a digital potentiometer. In one embodiment, the processor reads the temperature sensor and adjusts the potentiometer based on a correlative or predictive technique so as to provide a modified output that matches that of a standard resistive temperature probe and is compatible for display on a multiparameter monitor.

BACKGROUND

This invention relates generally to temperature probes used in themedical field and more particularly to temperature probes that areconnected to medical monitors used to measure conditions such as bloodpressure, oxygen content in the blood and body temperature.

Many medical monitors in use today have a port for connecting atemperature probe. This port is often used for connecting a resistivetype probe where the monitor measures the resistance of the temperatureprobe to determine its temperature and thus the temperature of thetissue that the probe is in contact with.

Temperature probes, when they come in contact with a body that is at atemperature different from themselves, generally take about threeminutes to adjust to the temperature of that body. The goal of someapplications is to continuously measure a patient's temperature over aprolonged period of time such as during an operation. Waiting for a fewminutes for a probe to come to temperature before an operation begins isacceptable but other temperature measurements require a faster responsetime. For example, when a practitioner is taking a patient's temperatureonce every hour it requires a quick response so the practitioner canacquire the temperature reading and then proceed to other duties.

Predictive type thermometers use techniques to determine or predict whatthe final stabilized temperature will be before the probe has reachedthermal stability. These types of thermometers generally show thepredicted temperature on an attached display and are usually notconnected to a monitor where other vital signs are taken.

Previous to this invention, a monitor that has been constructed for usewith a resistive temperature probe could not readily be adapted for usewith a predictive temperature probe. This is because the monitor isdesigned to receive a resistive input to indicate temperature whereasthe output from the predictive probe is a digital output of thetemperature. Thus, the output from a predictive probe cannot be input toa monitor designed to receive an input from a resistive probe.

This invention in one embodiment allows a predictive temperature probeto be used with multipurpose monitors that are designed to receiveresistive temperature inputs. A patient's temperature can be quicklytaken and recorded on the same device as other vital signs. In anotherembodiment, this invention allows the resistive output of a temperatureprobe to be modified to reflect other corrections and adjustments asdescribed herein in more detail.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the invention, the resistive outputof a temperature sensing device such as a thermistor is adjusted usingpredictive or correlative techniques and a modified resistive output isgenerated that is input to a multipurpose monitor. In one embodiment inparticular, the sensed temperature is input to a microprocessor thatdetermines a modified temperature reading that is translated in to adigital potentiometer setting required to achieve an output resistancethat corresponds to the modified temperature reading.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a temperature probe connected to a medicalmonitor in a conventional manner.

FIG. 2 is a diagram of a probe in accordance with one embodiment of theinvention connected to a medical monitor by means of an interface inaccordance with one embodiment of the invention.

FIG. 3 is a diagram of a probe having two thermistor outputs connectedto a monitor via an interface in accordance with another embodiment ofthe invention.

FIG. 4 is a diagram of probe and an interface that utilizes a fieldeffect transistor (FET) to modify resistance of a probe in accordancewith one embodiment of the invention.

FIG. 5 is a diagram of a probe and interface which utilizes a photocellto modify resistance of a probe in accordance with one embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference now to the drawings, and particularly to FIG. 1, there isshown a multiparameter medical monitor 1 with a sensor 2 connected toits temperature port in a conventional manner. The sensor 2 contains atemperature sensitive element such as a thermistor. The monitor includesconventional circuitry such as an ADC and a resistive bridge to monitorthe resistive signal received from the sensor and display thetemperature reading. The resistance of the thermistor located in the tipof the probe changes in relation to the temperature. The monitor detectsthe probe resistance to determine the temperature at any given time.

FIG. 2 illustrates a medical monitor 1 with a sensor 2 and an interfacein accordance with one embodiment of the invention. The interfaceincludes thermometer circuitry 4 such as an ADC and a resistive bridgefor obtaining a digital signal from the sensor. The output from thecircuitry 4 is input to a microprocessor 9. The microprocessor 9 mayemploy correlative or predictive techniques or algorithms to determine atemperature for reporting to the monitor 1. In one embodiment, themicroprocessor 9 executes a correlation algorithm or uses a look uptable to report a temperature to the monitor 1. For example, if thethermistor is being used to measure skin or temporal temperature, themicroprocessor may correlate the measured temperature with a temperaturesuch as internal body or core body temperature. In another embodimentthe processor may use a predictive algorithm to convert a temperaturereading taken shortly after the thermistor is placed, i.e., during aperiod of thermal instability, to a final predicted temperature beforethermal stability actually occurs so as to provide a more rapidtemperature reading. In any case, the temperature that is measured bythe probe is converted to a resistance output 6 that is input to themonitor 1 that corresponds to a modified or corrected reading that theclinician desires to monitor. The microprocessor 9 adjusts theresistance output from the sensor 2 by sending a signal to the digitalpotentiometer 8 that sets the resistance of the digital potentiometer 8such that the resultant resistance observed at the output 6 isindicative of the temperature that is to be displayed on the monitor asdetermined by the microprocessor. For example, a commercially-available1024-step digital potentiometer may be set by digital input from themicroprocessor to a value that corresponds to the resistance of aequivalent thermistor probe at the measured temperature. The interfacecircuit may use isolation devices and isolated power supplies topreserve the safety isolation of the monitor. In a particularembodiment, there will be no direct galvanic connection between themonitor and the interface circuit.

The present invention is particularly useful in conjunction with a YSI400 series temperature probe which has a single thermistor output. Inaccordance with this embodiment of the invention, the 400 series outputis modified by the microprocessor as illustrated in FIG. 2.

The invention is also useful to simulate the output of a YSI 700 seriestemperature probe. This probe is different than the 400 series probe inthat it includes two thermistors sandwiched together. As such, thisprobe includes two thermistor outputs. FIG. 4 illustrates a medicalmonitor 1′ with a sensor 2′ and an interface in accordance with oneembodiment of the invention. The interface includes thermometercircuitry 4′ such as an ADC and a resistive bridge for obtaining adigital signal from the sensor. The output from the circuitry 4′ isinput to a microprocessor 9. The microprocessor 9 employs correlative orpredictive techniques or algorithms to determine a temperature forreporting to the monitor 1′. In this embodiment, the interface includestwo digital potentiometers 8A and 8B and the microprocessor 9 adjuststhe resistance for each of the thermistor outputs by sending signals tothe respective digital potentiometers. The adjusted outputs 6A and 6Bare input to the monitor 1′. As a further manifestation of theinvention, an interface may be provided with two digital potentiometersthat can be used with a series 400 probe or a series 700 probe or theirequivalent. In this embodiment, when used with a series 400 probe, onlyone of the potentiometers would be adjusted whereas when used with aseries 700 probe, both would be adjusted.

A further embodiment of the invention uses a FET in place of the digitalpotentiometer to modify the resistive output and is illustrated in FIG.4. Temperature is measured with a sensor 51 and converted to digitalform using circuitry 52. A microprocessor 53 calculates the modifiedthermistor resistance as described above. A FET 54 is connected to theinput of the monitor 58, and the gate of the FET is controlled by ananalog output of the microprocessor 53. The source-drain voltage of theFET is measured with a high-impedance differential amplifier 57 andconnected to an analog input of the microprocessor 53. The sourcecurrent of the FET is measured by a low-value (e.g., less than 10 ohms)resistor 55 connected to the source terminal. The voltage across thisresistor is amplified by amplifier 57 and sent to the microprocessor 53.The microprocessor calculates current from the voltage reading, giventhe known value of the source resistor. The microprocessor divides thevoltage input by the current to get the equivalent resistance of theFET. This resistance is compared with the desired resistance and anydifference is applied as negative feedback to the FET gate. Thereforethe thermistor equivalent resistance can be obtained despite thenon-linear characteristics of the FET.

If the polarity of the monitor 58 is not compatible with the FETconfiguration shown in FIG. 4, those skilled in the art will recognizethat the FET may alternatively be connected in the reverse of theconfiguration illustrated in FIG. 4. Most FETs will function in thismode, although at lower gain. The feedback loop compensates for thislower gain. Furthermore, some monitors may apply pulsed or variablevoltages to the thermistor input. The microprocessor 53 may measure thepeak-to-peak voltages for these cases to obtain the voltage and currentreadings needed to compute the resistance. The interface circuit will beisolated from the monitor as described above using isolation devices andisolated power supplies to preserve the safety isolation of the monitor.For use with a monitor that is designed with inputs for more than onethermistor, the FET configuration is duplicated analogous to FIG. 3.

In a further embodiment illustrated in FIG. 5, a cadmium sulfidephotocell 65 is used in place of the FET in FIG. 4. Temperature ismeasured with a sensor 61 and converted to digital form using circuitry62. A microprocessor 63 calculates the modified thermistor resistance asdescribed above. A light-emitting diode (LED) 64 connected to amicroprocessor analog output is used to illuminate the photocell 65. TheLED current is adjusted to obtain the desired photocell resistance. Anegative feedback loop is used to compensate for the photocellnonlinearity as in the FET method. The current amplifier 67 and voltageamplifier 68 transmit current and voltage information to themicroprocessor to compute equivalent resistance of the photocell. Thephotocell is a non-polarized device, so there is no problem with reverseconnection to the monitor 69. For use with a monitor that is designedwith inputs for two thermistors, the LED/photocell configuration can beduplicated analogous to FIG. 3.

Other embodiments of this invention can be foreseen where thetemperature to be reported to the medical monitor is determined fromsome other means. The temperature could be reported to themicroprocessor via a serial port and then reported to the medicalmonitor via the resistive output.

1. An interface for a monitor and a temperature probe including atemperature sensor comprising: a logic circuit for determining amodified resistive output for the temperature sensor and a means forproviding the modified resistive output, wherein the means for providingthe modified resistive output is compatible with the monitor such thatthe monitor can display a temperature that corresponds to the modifiedresistive output from the temperature probe, said means including a FETcoupled to said logic circuit via a first terminal and via a feedbackarrangement, said means providing a FET resistance corresponding to themodified resistive output.
 2. The interface of claim 1 wherein the logiccircuit is programmed to execute a predictive or a correlativealgorithm.
 3. The interface of claim 2 wherein the logic circuit is amicroprocessor.
 4. A temperature probe comprising: a temperature sensorhaving a resistive output, a processor for determining a modifiedresistive output for the temperature sensor, the processor beingprogrammed to execute a predictive or a correlative algorithm, and a FETfor providing the modified resistive output in response to a signal fromthe processor, wherein the algorithm is a predictive algorithm thatconverts the resistive output of the temperature sensor during athermally unstable condition to a modified resistive outputrepresentative of a predicted temperature during a condition of thermalstability.
 5. The temperature probe of claim 4 wherein the processorexecutes an algorithm to convert the resistive output of the temperaturesensor to a modified resistive output that can be displayed by amonitor.
 6. The temperature probe of claim 4 wherein the probe includestwo FETs.
 7. A method for digitally modifying the resistive output of atemperature sensor which comprises inputting the resistive output fromthe temperature sensor to a logic circuit, implementing a predictive ora correlative algorithm using the logic circuit to determine a modifiedresistive output, controlling a gate of a FET to adopt a settingcorresponding to the modified resistive output, and outputting aresistance corresponding to the modified resistive output.
 8. The methodof claim 7 further including measuring a FET voltage with a firstamplifier, measuring a resistor voltage of a first resistor having afirst resistance, calculating a FET current using the first resistanceand the resistor voltage, calculating a FET resistance using the FETvoltage and the FET current, comparing the FET resistance to themodified resistive output and applying a difference between the FETresistance and the modified resistive output as a negative feedback tothe gate.
 9. A temperature probe comprising: a temperature sensor thatprovides a resistive output, a logic circuit for determining a modifiedresistive output for the temperature sensor, and a means for providingthe modified resistive output including a FET, wherein the logic circuitis a microprocessor programmed to execute a predictive or a correlativealgorithm, and wherein the microprocessor includes an output and the FETincludes a gate, where the output of the microprocessor controls thegate of the FET such that the FET provides a FET resistancecorresponding to the modified resistive output.
 10. The temperatureprobe of claim 9 wherein the microprocessor further includes: a firstinput from a first amplifier, where the first amplifier measures a FETvoltage of the FET, and a second input from a second amplifier, wherethe second amplifier measures a resistor voltage of a resistor having afirst resistance, where the microprocessor calculates a FET currentusing the first resistance and the resistor voltage from the secondinput, calculates a FET resistance using the FET voltage from the firstinput and the FET current, compares the FET resistance to the modifiedresistive output and applies a difference between the FET resistance andthe modified resistive output as a negative feedback to the gate.
 11. Aninterface for a monitor and a temperature probe including a temperaturesensor comprising: a logic circuit for determining a modified resistiveoutput for the temperature sensor and a means for providing the modifiedresistive output, wherein the means for providing the modified resistiveoutput includes a FET, and wherein the logic circuit includes an outputand the FET includes a gate, where the output of the logic circuitcontrols the gate of the FET such that the FET provides a FET resistancecorresponding to the modified resistive output.
 12. The interface ofclaim 11 wherein the logic circuit further includes: a second input froma second amplifier, where the second amplifier measures a resistorvoltage of a resistor having a first resistance, where the logic circuitcalculates a FET current using the first resistance and the resistorvoltage from the second input, calculates a FET resistance using the FETvoltage from the from the first input and the FET current, compares theFET resistance to the modified resistive output and applies a differencebetween the FET resistance and the modified resistive output as anegative feedback to the gate.
 13. A temperature probe comprising: atemperature sensor having a resistive output, a processor fordetermining a modified resistive output for the temperature sensor, theprocessor being programmed to execute a predictive or a correlativealgorithm, and a FET for providing the modified resistive output inresponse to a signal from the processor, wherein the processor includesan output and the FET includes a gate, where the output of the processorcontrols the gate of the FET such that the FET provides a FET resistancecorresponding to the modified resistive output.
 14. temperature probe ofclaim 13 wherein the processor further includes: a first input from afirst amplifier, where the first amplifier measure a FET voltage of theFET, and a second input from a second amplifier, where the secondamplifier measures a resistor voltage of a resistor having a firstresistance, where the processor calculates a FET current using the firstresistance and the resistor voltage from the second input, calculates aFET resistance using the FET voltage from the first input and the FETcurrent, compares the FET resistance to the modified resistive outputand applies a difference between the FET resistance and the modifiedresistive output as a negative feedback to the gate.